As offshore drilling and completion operations progress into deeper waters, especially in depths of water greater than 1000 feet, many relatively simple surface operations become complex and costly. One frequent operational requirement is that of engaging a hydraulic stab sub receptacle with a probe for the purpose of applying hydraulic flow and pressure to operate a function. The function can be a valve, blowout preventer, test port, or other such items.
These connections can be made by divers, by ROVs (Remotely Operated Vehicles) which are free swimming, or by manipulators which are guided into place.
These type operations have seen a history of field problems in that the force of insertion is somewhat unpredictable depending on a number of factors such as percent squeeze of the seals, surface finishes, shape of entrance chamfers, hardness of the seal members, cross sectional area of the seal members, and the outer diameter of the seal members.
Industry standards are being developed thru the American Petroleum Institute which should provide a maximum of 30-50 lbs. of insertion force, a number which will not allow the insertion of most stab subs manufactured at the present time.
A further complication to this is that if a significant amount of remote capability is to be gained, more than one stab sub needs to be engaged at an interface to allow multiple control functions. In some cases as many as ten functions are being considered at a time, each of which require hydraulic control thru a stab sub. The potential forces to be required to handle multiple insertions such as these are clearly in excess of the horizontal forces available from free swimming vehicles or divers.
A further complication to the remote stabbing tools is that for many of the subsea systems, a 20 year life expectancy is desired for the tooling. This means that the surface finish of the subsea receptacles can be questionable after prolonged exposure. Even when protectors have been in place for 20 years, the protected surfaces would be suspect.